Right in the beginning, long before the triangles are introduced, it's established that Cynthia may not always be faithful.The NTR, and swinging not really works without love, and romance content, so I don't really mind the dev didn't rush us at the start with too much cheating elements. I never said this game consists only of ntr contents, but the reason the triangle pin (NTR) is only an option is rather the NTR haters like you.
When the leasing contract for the cafe is signed Sergil says something along the lines of "I don't want to know what she did to convince the landlord."
I like the triangle idea.